Research in the R2G aims to harness light to power catalytic transformations

Our research interests lie in two areas of chemical space: (1) re-designing known reactions under milder reaction conditions and in a greener fashion and (2) discovering new reactions that expand the arsenal of transformations available for academic, industrial, and medicinal chemists. As such, we are interested in developing reactions wherein a photoredox catalyst, that is excited by visible light, works synergistically with a ground-state transition-metal catalyst to promote synthetically valuable transformations of enthalpically strong bonds. The mechanisms of these processes will be investigated such that the insights obtained can be leveraged to rationally design next-generation catalysts featuring heightened activities and selectivities. By accomplishing these goals, we can drive the development of new synthetic strategies that maximize the overall atom-efficiencies of multi-step synthetic sequences, which ultimately saves time, money, and precious limited resources. Researchers in our lab will gain extensive experience in organic and organometallic synthesis, Schlenk and dry box techniques, spectroscopic/crystallographic characterization of new molecules/complexes, development/optimization of new reactions, electro- and photochemical techniques, and data analysis.

Ongoing directions in the Romero group

Direction 1: Aminoboration reactions using isolable amino(pinacol)boranes as difunctionalization reagents. 

Aminoboranes (R2N-BR2) are molecules comprised of a nitrogen and boron atom covalently coordinated. The N–B bond is further strengthened by π-donation from the nitrogen lone pair to the empty p-orbital on boron leading to an enthalpically favorable, partially sp2-hybridized bond that is hard to cleave. Since their discovery, aminoboranes have captivated chemists across a variety of disciplines. From a synthetic organic chemistry perspective, aminoboranes are reagents for polymer synthesis, asymmetric catalysts, and borylation or amination reagents. Until recently, aminoboration reactions were limited to reactions of diboron and hydroxylamine reagents under copper catalysis. Although exciting in the synthetic expedience, the stoichiometric waste products generated sustainability concerns. In 2016, Prof. Romero (then a graduate student) developed a broadly applicable dehydrocoupling reaction of amines with hydroboranes (HBPin and 9-BBN) to access numerous aminoborane derivatives (ref 1). He then applied the products to the formation of aldimines that cannot be accessed through traditional routes (ref 2). Since that time, only a single bona fide aminoboration reaction with these molecules was reported using butyl lithium as an activating reagent. As such, the hunt for an aminoboration reaction under mild conditions remains afoot. To facilitate our discovery of the first reaction at this end, we undertook a study elucidating the reactivity trends of amino(pinacol)boranes in the presence of neutral Lewis bases (e.g., pyridines, amines, phosphines, etc.) and under electro-/photochemical conditions (ref 3). Our results show that all three strategies are viable avenues to activate aminoboranes. With that, members of our team are working to apply these critical insights to designing the first mild aminoboration reaction. Will YOU be the one to find the next great reaction?

Our contributions to the field of aminoboranes: 
1. E. A. Romero, J. L. Peltier, R. Jazzar, G. Bertrand. “Catalyst-Free Dehydrocoupling of Amines, Alcohols, and Thiols with Pinacolborane and 9-Borabicyclononane (9-BBN)” Chem. Commun. 2016, 52, 10563.
2. G. P. Junor, E. A. Romero, X. Chen, R. Jazzar, G. Bertrand. “Readily Available Primary Aminoboranes as Powerful Reagents for Aldimine Synthesis” Angew. Chem. Int. Ed. 2019, 58, 2875.
3. A. A. Braddock, G. E. Lee, E. A. Theodorakis, E. A. Romero*. "Interrogating Redox and Lewis Base Activations of Aminoboranes" Organometallics, 2022, 41, 3845.

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Direction 2: Light-driven and photoredox radical organic and organometallic arylation reactions

Aryl radicals are highly reactive fragments that can be harnessed to furnish a range of organic molecules. Significant research effort focuses on discovering new strategies to both access them from common reagents and deploy them in arylation reactions. While the most widely used aryl radical precursors are aryl diazonium salts, they are also explosive and should not be handled or stored in large quantities. This limitation obstructs their broad application to industrial scale reactions. Unlike diazonium salts, hypervalent iodine reagents like diaryliodonium salts (Ar2I) are bench stable, easily prepared, and robust arylation reagents. Aside from 2-electron arylation reactions, recent investigations have demonstrated photoredox radical generation strategies can be adapted to generate aryl radicals with elimination of innocuous Ar-I. Most procedures require the use of a photoredox catalyst (PC) to achieve radical generation, which can impact future sustainability as material scarcity increases. PC-free approaches to radical generation from Ar2I salts has been reported but each method has severe limitations. We recently discovered that simple Lewis bases under purple LED irradiation can generate aryl radicals from Ar2I salts in the simplest procedure discovered thus far. Then, using our new strategy, we dusted off a decade-old reaction that uses Pd to perform C-H arylation using aryl radicals generated via an Ir-based PC and adapted it to our PC-free conditions. In this way we showed that our conditions are amenable to reactions involving transition-metal-based catalysts. From here, we are curious to discover other reaction classes that would benefit from an easy aryl radical generation approach. Aside from using Ar2I salts as radical precursors, we are currently on the hunt for more sustainable precursors that do not eject half of the reagent as waste upon radical formation, but that is a story for another time. Stay tuned!

Our contributions to the field of aryl radical generation and use: 
1. Jonathan Galicia,† Nicholas R. McDonald,† Christopher W. Bennett, Jiajun He, Mark D. Glossbrenner, Erik A Romero*. "Exogenous Photocatalyst-Free Aryl Radical Generation from Diaryliodonium Salts and use in Pd-Catalyzed C–H Arylation" Chem Commun. 2024, 60, 6929. †Contributed equally

2. Christopher W. Bennett, Nicholas R. McDonald, Almeera Siddiqui,+ Erik A. Romero*. "Organophotoredox Pd-Catalyzed C–H Arylation at Room Temperature using Diaryliodonium Salts" ChemPhotoChem 2024, e202400285.

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earomero@ucsd.edu

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